You don’t have any Twitter-specific card markup on that page, but there is some OpenGraph og markup which is used as a fallback to generate a Twitter summary card. There’s no twitter:image or og:image tag in the page so there’s no way for Twitter to show an image in the card.

No, that content is not visible to me on that URL, either on the browser view source window, or via a command-line query using curl -A Twitterbot <url> as described in the troubleshooting post. You’ll need to figure out why there’s a difference. It could be that the wrong state is being cached somewhere, but we are unable to help you with that.